An example of exhaust gas treatment by means of a conventional exhaust gas treating system is explained with reference to FIG. 1.
In FIG. 1, reference numeral 1 designates a boiler; 2, a denitrator; 3, an air preheater; 4, a dust collector; 5, a gas-gas heater; 6, a desulfurizer; and 7, a stack.
As shown in FIG. 1, a denitrator 2 using a catalyst is installed at the outlet of a boiler 1 or the like, and an air preheater 3 is installed at the outlet of denitrator 2 so as to lower the temperature of the exhaust gas to about 130.degree. C.
The exhaust gas having passed through the aforesaid air preheater 3 is dedusted in a dust precipitator 4, passed through a gas-gas heater 5 and then introduced into a desulfurizer 6 where sulfur oxides (SO.sub.x) are removed therefrom. Thereafter, the exhaust gas is discharged into the atmosphere through a stack 7.
In order to remove sulfur oxides (SO.sub.x) from exhaust in the aforesaid desulfurizer 6, there has conventionally been employed the so-called lime-gypsum method in which the aforesaid sulfur oxides (SO.sub.x) are absorbed with the aid of calcium carbonate used as absorbing agent and recovered in the form of gypsum. In this method, attempts have been made to reduce the outlet concentration of sulfur oxides (SO.sub.x) by varying the gas-fluid ratio, the residence time and the like.
Usually, the concentration of sulfur oxides (SO.sub.x) in exhaust gas from boilers is in the range of 400 to 800 ppm, and it is intended in the aforesaid lime-gypsum method to reduce the outlet concentration thereof to 50-100 ppm.
However, recent environmental standards demand that the concentration of sulfur oxides (SO.sub.x) in exhaust gas should be reduced to a level of 5 ppm or less which is commonly known as a high-degree desulfurization level. In order to remove sulfur oxides (SO.sub.x) to a level of 50 to 100 ppm according to the aforesaid conventional lime-gypsum method, a marked increase in cost due to an increased size of equipment and the like is unavoidable, even though the conditions are optimized. Moreover, it is desired from the viewpoint of environmental problems to improve the efficiency of removal of sulfur oxides (SO.sub.x).
Furthermore, the aforesaid desulfurizer 6 employs the so-called lime-gypsum method in which sulfur oxides (SO.sub.x) present in exhaust gas are absorbed with the aid of calcium carbonate used as absorbing agent and recovered in the form of gypsum. This lime-gypsum method has the disadvantage of requiring a large amount of absorbing agent.
Among dry processes, only an adsorption process using active carbon has been put to practical use. However, this adsorption process uses water washing for the purpose of desorption and hence requires a large volume of water. Moreover, this process also involves problems concerning disposal of the resulting dilute sulfuric acid, drying of the adsorbent, and the like.
As described above, in the current practical process for the removal of nitrogen oxides present in exhaust gas from boilers, there is used a denitrator 2 based on the selective catalytic reduction (SCR) method in which nitrogen oxides are decomposed to nitrogen and water vapor by using a catalyst comprising V.sub.2 O.sub.5 supported on TiO.sub.2 and a reducing agent comprising NH.sub.3. However, this process involves the following problems. First, a reaction temperature of 300 to 400.degree. C. is required because of the performance of the catalyst. Secondly, NH.sub.3 is required for use as reducing agent. Thirdly, since the current leak level of NO.sub.x is from 5 to 40 ppm, an excess of NH.sub.3 needs to be injected for the purpose of reducing the leak level of NO.sub.x to zero.
Moreover, recent environmental standards demand that the concentration of nitrogen oxides (NO.sub.x) in exhaust gas should be reduced to a level of 1 ppm or less which is commonly known as a high-degree denitration level. In the aforesaid conventional denitration treatment based on the selective catalytic reduction (SCR) method, a marked increase in cost due to an increased size of equipment and the like is unavoidable, even though the conditions are optimized. On the other hand, it is desired from the viewpoint of environmental problems to improve the efficiency of removal of nitrogen oxides (NO.sub.x).
In view of the above-described problems, an object of the present invention is to provide an exhaust gas treating system which can treat exhaust gas at low temperatures without requiring any heating means and, moreover, can treat exhaust gas efficiently without using a large amount of absorbing agent.